I didn't mention comfort in the drawback of the T-line and Helix because both have this problem: both have relatively narrow tires (35mm for the T-line, the frame should accept 40mm but there is only one slick race tire in 40x349, 38mm for Helix, narrower for the Ultralight), Helix has bigger wheels but a stiffer frame+fork, T-line has a (poor) rear suspension and a softer frame (the titanium frame of the T-line is significantly softer than the steel main frame of the other Brompton).

So both are OK on good; smooth roads and bad on bad roads, cobbles for instance.

Testing the T-line in Europe is now relatively easy, Brompton has a now a network of retailers approved to sell and maintain the T-line (not all Brompton retailers are allowed to sell+maintain the T-line) most of these have a test T-line.